Nitride-bearing low carbon ductile steels



July 5, 1966 HAJIME NAKAMURA NITRIDE-BEARING LOW CARBON DUCTILE STEELS Filed Feb. l0, 1951 8 Sheets-Sheet 1 NNMSI M m Q a w w y m J7 o 1 M 0 s 7.., a w aa C O 4 .m u .il W/Sw o, C U .m Pm. A 6 C n@ 0 O M0. KJ o O a i8 0, .C m 8a lam C3 6 N.

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July 5, 1966 I-IAJIME NAKAMURA 3,259,488

NITRIDE-BEARING LOW CARBON DUGTILE STEELS Filed Feb. 10, 1961 8 Sheets-Sheet 2 ISO QUENCH &[`EMPERED CT/vIS) AIR. COOLED CT-I5J TRANSITION T I TEMPERATURE |00 I Cr/Is a Tfusafc /QENcH a TEMPEPED (TNS) AIR. cooLED (TAS) RELATION BETWEEN TRANSITION TEMPERATURES (T I5 l T 5) AND TOTAL AMOUNT OF ALUMINUM NITR-IDE AND ZIRCONIUM NITRIDE CONTAINED IN A NITRIDE-BEARJNG STEEL QUENcI-I a TEMPEEED l T-NoTcH cHARPY AIR- OI. IMPAcT VALUE CO ED Kg-hx/em' .os .Io AIN z/IN, RELATION BETWEEN v-NoTcI-I cI-IARPY IMPACT VALUE AND Torr-AL AMOUNT oF ALUMINUM NITRIDE AND zIRcoNIUM NITRIDE coNTAINEo IN A NITPJoE-EEARING sTEEI..

July 5, 1966 HAJIME NAKAMURA 3,259,488

NITRIDE-BEARING LOW CARBON DUCTILE STEELS Filed Feb. 10, 1961 8 Sheets-Sheet 5 Salz/bili@ CLJ/Ike of AZN rz dz/cile steel B.

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NITRIDE-'BEARING LOW CARBON DUCTILE STEELS Env/nem we C Enz/verdun C July 5, 1966 HAQIME; NAKAMURA 3,259,488

NITRIDE-BEARING LOW CARBCN DUCTILE STEELS Tem/verdure C United States Patent Oflice 3,259,488 Patented July 5, 1966 3,259,488 NlTRIDE-BEARING LOW CARBON DUCTILE STEELS Hajime Nakamura, Meguro-ku, Tokyo, Japan, assignor to Ishikawajima-Harima Jukogyo Kabnshiki Kaisha, Tokyo, Japan, a corporation of Japan Filed Feb. 10, 1961, Ser. No. 88,411 Claims priority, application Japan, Mar. 31, 1960, 5S/11,188, 35/11,189, 35/11,190

Claims. (Cl. 75124) This invention relates to nitrideabearing low carbon ductile steels.

More particularly this invention relates to new nitridebearing low carbon ductile steels containing from 0.015% to 0.090% by fweight of a stable nitride .selected from the -group consisting of beryllium nitride, :columbium nitride, a combination of aluminum nitride and beryllium nitride, a combination of aluminum nitride and columbinm ni- -tride, a combination of beryllium nitride and columbium nitride and a combination of said three aforesaid nitrides having a certain degree of solubility in the solid state steel at an elevated temperature rand/or from 0.01% to l0.10% by Weight of another kind of nitride selected from the :group consisting of titanium nitride, zirconium nitride and a mixture of both nitrides 4having practically no solubility in the steel either in the solid and molten states and further less than 0.35% by weight of carbon and if desired, less than 1% by weight of an alloying element selected from lthe group consisting of nickel, chromium, molybdenum, vanadium, manganese, and silicon.

I discovered .that a steel containing nitride -or nitrides, a part of which is dissolved in the matrix thereof and another part which exists as free nitride therein, is capable of precipitating out .dispersedly its nitride at its grain bound-aries and within its grains through .the -plastic working performed at an elevated temperature, and that such steel with the structure as described above possesses mechanical properties far superior to a steel with the same composition. Particularly, its low ,temperature toughness is remarkably improved and the transition temperature markedly shifted towards lower temperature.

IIn .the following, 'I will describe how a low carbon ductile steel with remarkably excellent properties may be produced by the present invention. 'References will be made to the drawing wherein:

fFIG. 1 shows the impact values and TrlS of a commercial killed steel at Various temperatures with respect to rthree states of as-rolled, as oil-cooled from 950 C. and as -air-cooled from 950 C.

YFIG. 2 shows the shear fracture ratio and TrS of the same steel,

iFIG. -3 represents the relations between the transition temperatures (TrlS and TrS `of a low carbon steel and its sum contents of aluminum nitride and zirconium nitride,

FIG. 4 shows the relation between the impact values of 'the same steel and its total -contents of nitrides,

FIG. 5 shows the solubility curve of aluminum nitride in `ductile steel B,

FIGS. 6 I('A), (B), (C), (D) present impact values and TrlS lof ductile steels A, B, C, D, respectively, in two different states Iof heat treatment of as oil-cooled from y950 C. and as air-cooled from 950 C.,

fFIGS. 7 (A), (-B), (C) and (lD) show shear fracture ratio and TrS of the same steels as FIGS. 6 (A), ('B), (C) and (ID), and

PIG. 8 shows the changes in mechanical properties of ductile steel as quenched from 1200 C. and reheated to various temperatures.

Here,the .terms Trl5 and TrS signify the .temperature at which the energy absorbed in a 2 mm. V-notch Charpy testing specimen reaches the value of 15 .ft-lbs. and the temperature at which shear-fractured portion of the same specimen occupies 50% of the cross-section area of the same specimen, respectively.

Lf a cast low carbon ductile steel, containing about 0.08% carbon, and 0.090% aluminum nitride precipitated at the grain boundaries in the form of large and long crystals, is subjected to Ia plastic deformation at a .temperature above the A3 transformation point by a working process such as tor-ging, rolling or like, the crystallitesV -of said steel in as-cast state are broken up and the nitrides present at .the grain boundaries may also be iinely subdivided, while the former crystallites, on the other hand, tend to grow coarser in general against the breaking action as Ithe .temperature is raised to a higher point.

However, the Igrowth of crystallites through the recrystall-ization process, as described above, is extremely hindered by the line hard nitride particles that are present within the struct-ure of said Ifinely dispersed steel, thus bringing about a iine crystalline structure to the said steel. It follows that, as the -cross-sectional area of the steel is steadily reduced, in other words, as .the rolling or forging ratio is increased, by repeated plastic Vdeformation at such an elevated .temperature las mentioned earlier,"

the crystallites of the said steel become lfiner, and furthermore, as the cnishing temperature of rolling or forging is lowered, particularly if such work is done at .a temperature range of -1l00 C. to 600 C., which corresponds 4to the maximum precipitation range of the aforementioned nitride, .the growth of recrystallized grains is even more inh-i-bited resulting in an even riiner :granular structure.

It was confirmed by Electronmicroscope technique and X-ray technique that these kinds of nitride that possess certain degrees of solubility in the solid state steel, when precipitated out suiciently, become as small as 0.1 micron or less.

Thus the strength at both :atmospheric and elevated temperature becomes greater While maintaining high impact values, particularly at low temperatures, and the ltransition temperature becomes extremely shifted to the lower temperature side.

In steels killed with titanium and zirconium in order to decrease the free nitrogen by forming stable compounds with ythe latter, the action of free nitrogen as a strength Iraiser is reduced and slip can .take place more readily, as a consequence of fwhich, the elongation'and reduction of area are remarkably improved and age hardening phenomenon is eliminated. Test results are next presented which show .the actual and real improvements in mechanical properties lof steels as -treated according to .the present invention in comparison with those Aof a commercial killed steel.

-Five kinds of steels were pre-pared tor the purpose, as

shown in the Table 1, of which steels A and iE contain` aluminum nitride only, steel B aluminum nitride and zircoruum nitride, steels C and D beryllium nitride.

TABLE 1 Chemical Component, Percent Steel C Si Mn P S N a Al Be Zr AlN BesNg A1203 Solole ZrN CbN Duotile Steel A O. 0.32 0.59 0. 016 O. 023 Ductile Steel B- 0. 08 0. 23 0. 57 0. 016 0. 021 Ductlle Steel C. 0. 09 0.34 0.60 0.013 0.025 Duotile Steel D 0.07 0.31 0. 65 0.015 0.022 Ductile Steel E 0.09 27 Du t'l St lF 0.09 .25

Killgetllestel 0. 13 0. 18 0. 79 0. 035 0. 031

The mechanical properties of these steels which have been treated at 950 C. for the precipitation of nitride and then cooled in air, are shown in the Table 2.

themselves under a stressed condition, thereby preventing the dislocations to ypile-up at grain boundaries leading to catastrophic crack formation thereat.

TABLE 2 Tensile Yield Yield Elongation, Reduction Steel Strength, Point, Ratio, percent of Area,

kgJmrn.2 kg./1nm.2 percent percent Ductile Steel A 45. 7-46. 8 33. 3-34. 3 72. 0-73. 5 35. 4-37. 5 67. 71. 9 Ductle Steel B. 41. 9-42. 2 3l. 5-33. 5 72. 6-79. l 39. 2-40. 7 77. 4-78. 4 Ductile Steel C 46. 5-46. 9 32. 3-36. 9 68. 7-78. 9 37. 8-40. 0 69. 8-73. 0 Ductile Steel 46. 3-47. 5 3G. 3-39. 3 76. 5-82. 7 35. 0-40. 5 69. 8J' 0 Ductile Steel E 43. 1-47. 3 32. 0-33. 8 70. 5-74. 0 37. 5-39. 0 71. 0-73. 5 Ductile Steel F. 43. 7 36. 2 82.8 38. 3 78. O Killed Steel 44. 0 30. 0 68.2 28. 0 57. 0

FIGS. 1, 2, 6 and 7 compare the sub-zero impact values, Tr and TrS of ductile steels due to the present invention with those of a commercial killed steel, in either air-cooled or oil-cooled condition.

The impact value at 0 C., Tr15 and TrS values of ductile steels, as read off these curves, are tabulated in Table 3.

TABLE 3 Impact Value, 1 Tr15, C. TrS, C.

kg.m./sq. cm. Steel AC 2 0C 3 AC OC AC OC Duetile Steel A 30. 0 32. 0 -100 -125 -65 -93 Ductile Steel B 36.0 38. 0 -110 -125 -82 -95 Ductile Steel C 33.0 33.0 95 -110 -85 -90 Duetile Steel D 32. 0 38.0 -100 -110 -75 -93 Ductile Steel E 32. 0 37. 0 -70 -75 -40 -45 Ductile Steel F 32.0 4 38.0 -100 4 -120 80 4 -90 Killed Steel 15. 0 16. 0 57 -70 -35 -40 l Charpy -notch, at 0D C. 2 Air-cooled. 3 Oil-cooled.

4 Oil-cooled from 930 C. and tempered to 650 C.

FIGS. 3 and 4 show the relation of the total quantity of nitrides to the impact value, Tr15 and TrS, It may be clearly seen that a larger amount of total nitride always corresponds to a higher impact value and lower TrlS and TrS. It may also be observed that for a steel containing a total amount -of nitride over 0.03% up to 0.10%, the low temperature properties are markedly improved.

The result of rupture tests run at 450 C. on ductile steel A indicated a range of duration time of between 88 hours to 125 hours under a stress of 20 kg./rnrn.2 with an elongation of between 70% to 80% at rupture, whereas steels with comparable composition but containing no nitride would fail, `as judged by the American Society for Testing Materials specifications, after about only 50 hours with an elongation of about to 40% at rupture. This result may be regarded as demonstrating the excellent properties at la high temperature of ductile steels of the present invention.

This exceptionally good high temperature strength may be attributed to the nitride particles which are present in finely dispersed form, in a size as small as 0.1 micron or less, on slip planes of crystallites that restrict the free movement of dislocations by anchoring the latter around It may be seen from the above experimental results that nitrides are `dispersedly precipitated at the grain boundaries and within the grains, and that steels with such a structure exhibit mechanical properties at both atmospheric and elevated temperatures that are far superior to steels of comparable chemical composition but containing no nitride. The improvement in low temperature properties of such nitride-bearing steels is also particularly remarkable.

The present invention relates also to a method of heat treatment for the aforementioned steels containing at least one kind of metallic nitride as described hereinbefore whichcomprises heating the said nitride-bearing low carbon steel to a temperature above the A3 transformation point and within such a temperature range that the precipitation of said nitride takes place maintaining at said temperature for a sufiiciently long time, so that a large amount of precipitation of said nitride is obtained. The nitride is either aluminum nitride, beryllium nitride, columbium nitride or any combination thereof. Thereafter the said steel is rapidly cooled in air, water or oil while subsequent to the aforementioned process of cooling, the said steel may once again be heated to a temperature below the transformation point for tempering the steel, in order to provide a method of heat treatment for a nitride-bearing ductile steel which not only improves the mechanical properties of the said steel at both room and high temperatures by refining the grain size thereof, but also increases the toughness thereof, particularly `at low temperatures, and makes the transition temperature shift towards the lower temperature side.

The heating temperature and holding time may be Varied depending on the composition of the steel to be treated, but it is generally preferable to maintain the steel at a temperature -above the A3 transformation point, namely between 800-1100 C., for a period of time more than 30 minutes.

The tempering is preferably conducted at ture between -700 C.

The present heat-treatment method is more fully understood by reference to the drawing.

For example, for a pure iron, low carbon steel or low carbon low alloy steel containing 0.020-0.045% nitrogen, 0.0400.5% aluminum, 0.05-0.3% zirconium or 0.030- 0.1% titanium or both, there may be employed a process comprising heating the said metal to a temperature bea temperatween 800 C. to 1100" C. for 30 minutes and, after precipitating the metallic nitride out to the maximum extent, quenching it from a temperature above the A3 transformation point in air, in oil or in water and subsequently,

the A3 transformation point, it follows then that a line grained steel with excellent mechanical properties can be obtained more advantageously by employing both of these processes mentioned above simultaneously rather if desired, tempering at a temperature within a range of 5 than by any one of these processes as used singly. That 150-700 C. is to say, the present invention relates to a heat treat- For a steel containing 0.005-0.045% nitrogen, 0.01- ment method for nitride-bearing steels in which `a simul- 0.20% beryllium and 0.05-0.20% zirconium orl 0.03- taneous employment of both of these processes described 0.20% titanium, the process may involve heating at a above is featured. temperature between 800 C.-l100 C. for less than one 10 Herefollowing, practical examples of the said method (1) Ihour and then quenching from a temperature above as applied to four (4) kinds of representa-tive ductile the A3 transformation point in air, in oil or iin water, steels will be described in detail to show how a nitrideand further, if preferred, tempering at a temperature bebearing low carbon steel is made to change, improving tween 150-700 C. its structure (ferrite crystal size), room temperature In order to show that the mechanical properties are mechanical proper-ties (tensile strength, yield point, improved by the process of heat treatment due to the elongation, reduction of area, hardness, impact -value or present invention, which involves rapidly cooling or like), sub-zero impact values, Tr15, and TrS. Table 4 quenching a nitride-bearing ductile steel in a temperawhich contains a part of Table l shows the chemical comture range of most enhanced precipitation for nitrides positions of said steels.

TABLE 4 l C l si Mn 1 1? l S A1 l Be Zr N2 AlN BeiNi ZrN Ducale steelA 0.10 l0.32 0.59 0.016 0. 023 0.096 0. 023 0.071 Ductile stee1B 0.08 0. 2s 0.57 0.016 0.021 0. 085 0.06 0. 0:12 0. 087 0.02 Ductile Steele-- 0.09 0. 34 0.60 0. 013 0.025 0.006 0.119 0.009 0.016 Ductile stee1D which lies above the A3 transformatiton point, the fol- It lhas been established for all the said steels that the lowing is presented, iirst there is shown the test results ferrite grain size becomes smaller as the cooling rate is of the solubility of nitride in solid state steel, then exincreased in order of air-cooling, oil-cooling and wateramples of heat treatment for representative nitride-bearcooling. ing ductile steels are shown. Namely, nitride-bearing duc- Table 5 presents the grain size of these steels as extile steel containing 0.032% nitrogen was heated for pressed in terms of lthe ASTM grain number. It may be two (2) hours at 1200" C. in order to dissolve the ni- 35 seen that extremely tine grain size was attained which tride into matrix, then the said nitride was reprecipitated hitherto was not possible with a mild steel. out by heating the said steel at various temperatures be- TABLE 5 tween 200 and ll00 C. for one (l) hour at respective temperatures, and then the nitride was chemically separated from the matrix and the amount of nitride was Steel ASTM Ferme GramNumber measured. The value thus obtained is to be interpreted A Y as representing a sum of coarser nitride which had al- C OC C ready been precipitated out at solid state, namely, that which is insoluble at 1200 C., and liner ones which were ii" 1g 5 a newly precipitated in the grains. Therefore, the greater gg gj 1g the value, the more the yamount of nltlldes that have ben newly precipitated in the gain isindicaedf FIG It is to be noted that, in each of steels, the ferrite 5 illustrates 'the results of experiments conducted a grain number becomes larger in an Order of aipcooling, steel contalnlng AlN. It may be seen that the poreclplta- 0i1 c0o1ing and Waterooling '1011 )f nltffde reaches a max'lmum at about 700 and Table 6 which is ya part of Table 2 shows the tensile remains fairly CClUSaUf UP t0 1100 C afef Which il strength, yield poit, elongation, reduction of area, and declines rather rapidly on account of redissolution of Charpy v notch impact Value at 40 Q of the airthe precipitated nitride into the matrix. cooled steels.

TABLE 6 Tensile Yield Reduction Impact Steel Strength, Point, Elongation of Area, Value* lig/mm.2 kg./rnm.2 percent kg.-n1./cm.2

Ductile SteelA 45.74166 sas-34.3 35. 4-375 67. 571.9 30.0 Ducale steelB 419-422 31. 5-3a5 39. 2410.7 77. 448.4 36.0 Ducale Steele 46.5.46.9 sas-36.9 37.84100 69. s-7a0 33.0 Ducale steelD 46a-47.5 :16a-39.3 aaO-40.5 69.s73.0 30.7

*Charpy 2 mm. V-notch, at 40 C., in kg.-m.]cm.2.

Similar relationships were also observed on steels containing beryllium nitride.

Therefore, when the improvement of the mechanical properties of steels is to be realized by virtue of the precipitation of nitride only, this may be achieved by heating the said steel to a temperature below the transformation point for a suticiently long period of time, but as the granular structure of a steel can be refined by Of these steels, one with less than 0.01% carbon, namely a very mild steel, exhibits almost no change by air-cooling or oil-cooling thereby needing no tempering, yet, such a steel, too, can be made ductile by the method of the present invention so as to be a structural steel in suciently low temperatures.

FIGS. 6 and 7 represent the comparison of sub-zero impact values, Tr15 and TrS between representative ducrapidly cooling it from a temperature immediately above tile steels described above as obtained according to aircooled and oil-cooled operations, respectively, to show that, in any one of the cases, oil-cooled steels exhibit better properties than those that are air-cooled. Though no particular emphasis is needed, it may be stated here that these steels in merely an air-cooled state are superior to those in as-annealed, vas-rolled or as-forged conditions.

The above results are summarized in the following Table 7 which is a part of Table 3.

*Maximum value, Charpy 2 mm. V-notch, at C. TOil-coolcd and tempered.

Steels containing more than 0.10% carbon tend to harden somewhat by quenching, and as a result, the tensile strength, yield point, and hardness are slightly raised, While the elongation, reduction of area and impact values suffer a reduction. This means the process of tempering become necessary for such steels following the watercooling. FIG. 8 shows the change in tensile strength, yield point, hardness, elongation, reduction of area and impact value of ductile steel .A as the said steel is tempered at various temperatures above 200 C. following quenching from 1100 C.

It may be seen that the tensile strength and yield point are reduced rather sharply at 400 C.; the elongation is almost recovered at -about 300 C. and is improved still further as the tempering temperature is raised; while, the reduction of area and impact value are completely recovered at about 300 C. and no noticeable .further gain is observed at `a higher temperature.

It may be stated on the strength of these experimental results described above that the mechanical properties of ductile steel can be improved by rapidly cooling the said steel from a temperature within the range of maximum nitride precipitation and above the A3 transformation point, and lthat such steel becomes an excellent structural steel for low temperature service with its Tr15 and TrS, both of which are known to represent well the low temperature ductility, and are markedly shifted tomore stable nitrides, in order to convert the excessive nitrogen into nitride or nitrides of the aforementioned metal or metals, and if desired, heat treating the steel thus produced in order to precipitate out a part of the nitride, and then subjecting the steel to a plastic deformation at an elevated temperature such as by rolling or forging.

The steels utilized in this process are pure iron, low carbon steels with approximately 0.35% carbon or less, or low carbon low alloy steels containing alloying ele- 10 ments other than carbon by less than 1.0% each.

I have found that when from 0.015 to 0.09% of nitride or nitrides of metal or metals such as aluminum, beryllium, colurnbium or the like are present in a steel,

the said steel possesses superior mechanical properties at v atmospheric and elevated temperatures, particularly as regards its impact strength at low temperatures while the transition temperature is remarkably shifted towards the lower temperature side, as compared to a steel with the same components except the nitride.

The nitride-bearing steel may be successfully produced using any one of an over-blowing converter, open-hearth furnace, electric arc furnace, or high-frequency electric furnace and the production method can be categorized into two categories, as now will be described more fully in the following.

subsequently adding to the steel one, or more than one kind of a metal that reacts with the nitrogen to form a hard nitride having a certain degree of solubility in the solid state steel, such metals may be aluminum, beryllium, or columbium. These metals react with the aforementioned nitrogen, making the said steel a nitride-bearing ductile steel. One practical example of producing ductile steel in an electric arc furnace by means of a process mentioned above is now described in detail.

Example I Firstly, scrap steel was melted by an electric arc furnace and refined under an oxidizing condition, then 0.35% of ferro-silicon and 0.70% of ferromanganese were added for deoxidation, then nitrogen gas was blown into the molten steel through a conduit pipe for about 6 minutes at a pressure of 5 kg., following which 0.20% of aluminum in one case and 0.10% beryllium in the other was added prior to the casting of the steel into ingots. The Table S shows the chemical composition of two (2) representative steels produced by the aforementioned wards the lower temperature side. process.

TABLE 8 steel C si Mn P s N2 l A1 AlN A1203 Be BeNz Nitride-bearing low carbon steels of this invention can be produced by one of the following three processes. Thus, the present invention 4relates also lto a method of production of nitride-bearing low carbon steels which comprises blowing into molten steel nitrogen gas or a mixed gas composed of nitrogen and a gas that is inert to the steel or blowing calcium cyanamide into the melt with nitrogen or inert gas or a mixture gas thereof during the reduction period of the steel-making process to render the molten steel nitrogen-bearing, and subsequently adding at least one metal element such as aluminum, beryllium, columbiurn or the like that combines with nitrogen to form a metallic nitride that possesses a certain degree of solubility in the solid state steel, to be nitrided by the said nitrogen within the aforementioned molten steel, Iand thereafter, adding, optionally and furthermore, zirconium or titanium or both, that yield even As may be seen, a part of the aluminum added is combined with oxygen that was present in the molten steel and becomes aluminum oxide, while the rest is formed as aluminum nitride. Also, the beryllium nitride, Be3N2, was clearly shown by the chemical analysis.

Nextly, ingots thus produced were brought to a temperature between 1150 C. and 1250" C., which corresponds to a state where the nitride exists partially as dissolved and partially as solid phase, and the ingots were forged thereat with a finishing temperature 850 C. The steel was left to cool to atmospheric temperature, subsequent to which the said steel was heated again at 950 C. for one (l) hour, and then again cooled in air. Table 9 compares the mechanical properties of steels produced by the above method and of a commercially available high killed mild steel with a comparable chemical composition.

TABLE 9 Tensile Elonga- Reduction Impact Steel Strength, Yield Point, tion, of Area, Value,2 Tr15, TrS, kg./sq. mm. kg./sq. mm. percent percent kg.1n./ C. C.

sq. cm.

42. 0 32. 0 40. 0 79. 0 35. 5 -105 -85 H 46. 7 38. 0 39. O 71. 0 31. 0 -100 -70 Mild Steel 1 44 30 28 57 8. 0 -47 -30 1 0.13% C, killed. 2 At C.

Therefore, it can be stated that the present method is fully capable of producing a nitrogen-bearing ductile steel.

Furthermore, this process of nitrogen blowing has an additional effect of obtaining cleaner steel by virtue of the stirring action the nitrogen gas induces within the molten metal, thus stimulating various deoxidization products that are present therein to coagulate.

If a nitride-bearing steel is to be produced by the aforementioned process only, the ingot is not stabilized, or killed, and becomes a so-called rimmed ingot containing blow holes within, since an amount of nitrogen gas that is left uncombined tries to escape therefrom as the metal solidilies. However, even this kind of ingot provides no disadvantage when offered for sale, as far as the quality of the steel is concerned, since the aforementioned blow holes can readily be eliminated during forging or rolling.

However, killed steels are often more desirable when higher quality ingots are desired. In order to achieve this purpose, it calls for use in a general practice of a quantity of aluminum as much as about 1%, when used singly to bring forth a change in the properties of the steel.

I have succeeded -in stabilizing, or killing, the nitridebearing ductile steel by fixing the excessive nitrogen gas that is escaping therefrom at its solidiication by means of a metal or metals that can form even more stable nitride or nitrides than those of aluminum or beryllium and that do not dissociate at a solidilication temperature of the steel. Of metals with the aforementioned property, titanium and zironium were found to be most suitable, and an addition of about 0.03% to 0.3% thereof was seen to be suicient to achieve the purpose. The titanium nitride and zirconium nitride resulting herein are all insoluble in the molten steel, one part of which is eliminated therefrom, while .the rest which is floating therein, become nely and dispersedly suspended therein as the steel solidifies; the particles of said metallic nitride in a form as described above have no ladverse effects on the steel, and even a favorable influence could be expected for the steel.

In Table l0 are represented the chemical composition and mechanical properties of a steel produced through the process described above.

The second method comprises blowing the calcium cyanamide into the molten steel together with an inert gas (for example, argon) or nitrogen gas or a mixed gas consisting of nitrogen and an inert gas, so that nitrogen be introduced thereinto as a result of direct reaction of gaseous nitrogen, and subsequent to the above, an amount of metallic element or elements that yield the desired nitride as before is introduced, thus rendering the steel nitride-bearing and ductile.

I therefore particularly point out and distinctly claim as my invention:

1. A nitride-bearing ductile steel consisting of less than 0.35% carbon, less than 0.035% of each of sulphur and phosphorus, less than 1% of manganese, less than 0.5% of silicon, from 0.015 to 0.090% of at least one of the substances selected from the group consisting of beryllium nitride, columbium nitride, a Combination of aluminum nitride and beryllium nitride, a combination of aluminum nitride and columbium nitride, a combination of beryllium nitride and columbium nitride, and a combination of aluminum nitride, beryllium nitride and columbium nitride; from 0.01 to 0.10% of at least one of the nitrides selected from the group consisting of titanium nitride and zirconium nitride, the remainder being substantially all iron with incidental impurities, a portion of the nitrides being combined in the matrix of the steel, the remainder of the nitrides existing as free nitride in the form of line particles dispersedly precipitated in the grains and on the grain boundaries of the steel to thereby provide improved mechanical properties for the steel.

2. A nitride-bearing ductile steel consist-ing of less than 0.35 carbon, less than 0.035% of each of sulphur and phosphorus, less than 1% of manganese, less than 0.5 of silicon, from 0.015 to 0.090% of the substances selected from the group consisting of; beryllium nitride, columbium nitride, a combination of aluminum nitride and beryllium nitride, a combination of aluminum nitride and columbium nitride, a combination of beryllium nitride and columbium nitride, a combination of aluminum nitride, beryllium nitride and columbium nitride, from 0.01% to 0.10% of at least one of the nitrides selected from the group consisting of titanium nitride and zirconium nitride less than 1.0% of at least one of the elements selected from the group consisting of nickel,

TABLE 10 Steel C Sl Mn P S Ng Al Zr AlN A1205 ZrN Tensile Yield Reduction Impact Steel Strength, Strength, Elongaton, of Area, Value,i' Tr15, TrS, lig/sq. mm. kg.lsq. mm. percent percent kg.m./sq. C. C.

* uctile steel. iAt 0 C.

It may be seen that the xation of free nitrogen by means of adding such metallic element like zirconium or titanium is evidently elective for producing killed ductile steel.

chromium, vanadium, and molybdenum; less than 0.5% boron, the remainder being substantially all iron with incidental impurities, a portion of the nitrides being combined in the matrix of the steel, the remainder of the nitrides existing as free nitride `in the form of fine particles dispersedly precipitated in the grains and on the grain boundaries of the steel to thereby provided improved mechanical properties for the steel.

3. A nitride-bearing ductile steel consisting of less than 0.35% carbon, less than 0.5% silicon, less than 1.0% of at least one of the elements selected from the group consisting of manganese, nickel, chromium, vanadium, and molybdenum, less than 0.5% boron, from 0.015 to 0.090% aluminum nitride and from 0.01 to 0,10% of at least one of the nitrides selected from the group consisting of titanium nitride and zirconium nitride, the remainder being substantially all iron with incidental impurities, a portion of the nitrides being combined in the matrix of the steel, the remainder of the nitrides existing as free nitride in the form of fine particles dispersedly precipitated in the grains and on the grain boundaries of the steel to thereby provide improved mechanical properties for the steel.

4. A nitride-bearing ductile steel consisting of less than 0.35% carbon, less than 0.35% of each of sulphur and phosphorus, less than 1% of manganese, less than 0.5% of silicon, from 0.015 to 0.090% of at least one of the substances selected from the group consisting of beryllium nitride, columbium nitride, a combination of aluminum nitride and beryllium nitride, a combination of aluminum nitride and columbium nitride, a combination of beryllium nitride and columbium nitride, and a a combination of aluminum nitride, beryllium nitride and columbium nitride, the content of beryllium nitride being from 0.015 to 0.065%; from 0.01 to 0.10% of atleast one of the nitrides selected from the group consisting of titanium nitride and zirconium nitride, the remainder being substantially all iron with incidental impurities.

5. A nitride-bearing ductile steel consisting of less than 0.35% carbon, less than 0.035% of each of sulphur and phosphorus, less than 1% of manganese, less than 0.5 of silicon, from 0.015 to 0.090% of at least one of the substances selected from the group consisting of beryllium nitride, columbium nitride, a combination of aluminum nitride and beryllium nitride, a combination of aluminum nitride and columbium nitride, a combination of beryllium nitride and columbium nitride, and a combination of aluminum nitride, beryllium nitride and columbium nitride, the content of columbium nitride being from 0.035 to 0.090%; from 0.01 to 0.10% of at least one of the nitrides selected from the group consisting of titanium nitride and zirconium nitride, the remainder being substantially all iron with incidental impurities.

References Cited by the Examiner UNITED STATES PATENTS 2,174,740 10/1939 Graham et al 75-123 XR 2,482,948 9/1949 Sykes et al. 75129 2,501,138 3/1950 Parker 75-129 2,562,543 7/1951 Gippert 75-123 2,563,672 8/1951 Boyce 148*134 2,602,736 7/1952 Sheridan 75-123 2,603,562 7/1952 Rapatz 75-123 2,938,820 5/1960 Turner 148-134 FOREIGN PATENTS 530,500 8/1955 Italy.

DAVID L. RECK, Primary Examiner. RAY K. WINDHAM, MARCUS U. LYONS, Examiners. R. O. DEAN, Assistant Examiner. 

1. A NITRIDE-BEARING DUCTILE STEEL CONSISTING OF LESS THAN .035% CARBON, LESS THAN 0.035% OF EACH OF SULPHUR AND PHOSPHOROUS, LESS THAN 1% OF MANGANESE, LESS THAN 0.5% OF SILICON, FROM 0.015 TO 0.090% OF AT LEAST ONE OF THE SUBSTANCES SELECTED FROM THE GROUP CONSISTING OF BERYLLIUM NITRIDE, COLUMBIUM NITRIDE, A COMBINATION OF ALUMINUM NITRIDE AND BERYLLIUM NITRIDE, A COMBINATION OF ALUMINUM NITRIDE AND COLUBIUM NITRIDE, A COMBINATION OF BERYLLUM NITRIDE AND COLUMBIUM NITRIDE, AND A COMBINATION OF ALUMINUM NITRIDE, BERYLLIUM NITRIDE AND COLUMBIUM NITRIDE; FROM 0.01 TO 0.10% OF AT LEAST ONE OF THE NITRIDES SELECTED FROM THE GROUP CONSISTING OF TITANIUM NITRIDE AND ZIRCONIUM NITRIDE, THE REMAINDER BEING SUBSTANTIALLY ALL IRON WITH INCIDENTAL IMPURITIES, A PORTION OF THE NITRIDES BEING COMBINED IN THE MATRIX OF THE STELL, THE REMAINDER OF THE NITRIDES EXISTING AS FREE NITIDE IN THE FORM OF FINE PARTICLES DISPERSEDLY PRECIPITATED IN THE GRAINS AND ON THE GRAIN BOUNDARIES OF THE STEEL TO THEREBY PROVIDE IMPROVED MECHANICAL PROPERTIES OF THE STEEL. 